70X mods to fix SiStrip Tracker PreMixing

Add special 10-bit FED Readout Mode in simulation to preserve high pulse height hits in Tracker through the Digi to Raw step.  Both Packer and Unpacker were modified to allow this (A.-M. Magnan).  Added a new ZeroSuppression mode that merely passes any adc > 0 so that low-level hits are not lost.  Because of all of this, the SiStrip Digitizer now needs to know that it is working in PreMixing mode, so as to not truncate all hits at ADC=256, so this switch was added. One customization added to ConfigBuilder so that the new PreMixing readout mode is enabled in the PreMixing DigiToRaw step.
Yes, I agree.  This was the original structure, though, so we just added another block of code.